1. Field of the Invention
The present invention relates to an apparatus for measuring a thickness of a film to be measured and a method for measuring a thickness of a film to be measured using an electron beam.
2. Related Art
In the past, for example in the method disclosed in Japanese unexamined patent publication (KOKAI) No. H6-273297, when making a thickness of a sample to be thin so as to give the sample a thin film like configuration by irradiating an ion beam at the sample, an electron beam is irradiated at the sample simultaneously with the irradiation of the ion beam thereat, and the electron beam having passed through the sample being detected by a Faraday cup so as to prevent excessive etching against the sample.
In Japanese unexamined patent publication (KOKAI) No. H8-5528, there is a disclosure of a technology in that an ion beam machining device is used for making a sample to be used for a transmission electron microscope and when an electron beam is shone onto a location to be processed thereby, an amount of current of the electron beam passing through the processed part is detected so as to control the amount of ion beam processing.
In these methods disclosed in Japanese unexamined patent publications (KOKAI) Nos. H6-273297 and H8-5528, however, the sample to be measured must be thinned to a thickness through which the electron beam can pass, making it difficult to measure the film thickness of a thin-film formed on the supporting substrate such as in a conventional semiconductor device.
The inventors of the present invention, in order to solve the above-noted problem in the related art, has succeeded in developing a technology for calculating the film thickness of the thin-film to be measured by measuring a value of the substrate current flowing in the substrate when an electron beam is caused to strike or irradiate a thin-film formed on a supporting substrate, and calculating the film thickness of the thin-film based on reference data, and the applicant had already filed a patent application for this technology (Japanese unexamined patent publication (KOKAI) No. 2000-180143).
In this method, rather than measuring an amount of the electron beam passing through the sample, but a substrate current value is measured directly from the substrate, and thus it is possible to measure the thickness of even a thin-film formed on the supporting substrate.
FIG. 1 of the accompanying drawings is a block diagram showing a film-thickness measurement apparatus as disclosed in the Japanese unexamined patent publication (KOKAI) No. 2000-180143. This apparatus has an electron gun 3 shining an electron beam onto a thin-film 2 on a substrate 1, an electrode 4 disposed in contact with the bottom part of the substrate 1, and a current measuring part 5 that measures the substrate current value collected at the electrode 4.
The current measured at the current measuring electrode 5 is adjusted by a current amplifier 6 and a differential amplifier 7, and is converted to a digital signal by an A/D converter 9.
The film-thickness measuring apparatus further has a measured current storage part 10 that stores the measurement current value that has been converted to a digital signal, a calibration curve data storage part 11 that stores calibration curve data measured using an existing standard sample, and a calibration curve data comparator 12 that compares the calibration curve data with a measured current value.
A film-thickness measuring apparatus configured as described above has the effect of enabling measurement of the film thickness of films and further is effective for thin film and is more effective for ultra-thin films.
The invention noted in the Japanese unexamined patent publication 2000-180143 uses the following principle.
When a low-energy electron beam of between several hundred keV and several keV is made to strike a sample, secondary electrons are emitted from the region in the vicinity of the surface of the sample.
In general, conductors and semiconductors generally have a low capacity for emitting secondary electrons, while insulators have a large capacity for emission secondary electrons.
For example, in contrast to the secondary electron emission capacity of 0.9 or so of silicon, which is a semiconductor, a silicon oxide film, which is an insulator, has a secondary electron emission capacity of approximately 2.
Therefore, when an electron beam is caused to be irradiated at a semiconductor device in which a thin-film made of a silicon oxide film is formed on a surface of the silicon substrate, more secondary electrons are emitted from the silicon oxide film.
When this occurs, electron flow into the silicon oxide film from the silicon substrate to compensate for the secondary electrons emitted from the silicon oxide film.
That is, the substrate current that is a sum of the current arising because of the electron beam striking the silicon substrate and the compensating current flowing the substrate in the direction opposite thereto, flows through the silicon substrate.
FIG. 2 is a schematic representation of the above-noted principle.
As shown in FIG. 2(a), in the case in which a thin-film made of silicon oxide is formed onto a silicon substrate, if when one electron from an electron beam strikes the surface thereof, two electrons are emitted from the silicon oxide film as secondary electrons.
Because one electron is emitted from the silicon oxide film, in order to compensate for the electron emitted from the silicon oxide film, one electron flows into the silicon oxide film from the silicon substrate.
In this case, a substrate current flows in the silicon substrate in the direction opposite to that of the current due to the electron beam.
On the other hand, as shown in FIG. 2(b), in the case in which there is no silicon oxide film on the silicon substrate, when one electron beam electron strikes on the surface of the substrate, 0.9 electron is emitted from the silicon substrate as secondary electron emission.
For this reason, a substrate current having an amount formed by subtracting the amount of electrons as released from the amount of electrons thus striking the substrate, flows in the substrate in the direction identical to that of the current arising by the electron beam.
As described above, because there are few secondary electrons emitted when there is no silicon oxide film on the silicon substrate, the major portion of the current is attributed to the electron beam, but as the film thickness of the silicon oxide film increases, the compensation current increases.
However, because there is an increase in the resistance value of the silicon oxide film, which is an insulator, when the thickness of the silicon oxide film increases further, it becomes difficult for the compensation current to flow, so that there is a reduction in the substrate current.
FIG. 3 shows graphs illustrating the relationship between the substrate current value and film thickness.
As shown in FIG. 3(a), if a current in the direction of the current caused by the electron beam is taken as being a positive current, until the thickness of the silicon oxide film reaches a prescribed thickness d, there is a increases in the negative direction, after which the substrate current value decreases and converges to zero.
Because the Japanese unexamined patent publication (KOKAI) No. 2000-180143, deals with the measurement of gate oxide films or the like, which have a relatively small thickness of several nanometers, such as a super thin film, as shown in FIG. 3(b) there is a 1:1 relationship established between the substrate current value and the film thickness of the thin-film.
However, at large thickness in the range from several tens of nanometers to several thousands of nanometers, because the relationship between the substrate current value and the film thickness becomes 1:2 as shown in FIG. 3(a), it is discovered that there is a problem in that the measured substrate current value does not have a unique thickness of the thin-film when the substrate current is measured.
The above-noted problem remains in measuring a thickness of not only ultra-thin films but also of films having a wide range of thickness by applying the method disclosed in the Japanese unexamined patent publication (KOKAI) No. 2000-180143.
The present invention provides an improvement relative to the technology disclosed in the Japanese unexamined patent publication (KOKAI) No. 2000-180143 so as to solve the above-noted problem.
Accordingly, it is an object of the present invention to provide technology for measuring the film thickness of a film including a thin-film, formed on a substrate, and particularly to provide a technology for measuring the film thickness of films having a wide range of thickness.